I agree that Gin-san just acts like an idiot, he actually seems quite insightful sometimes. Plus I think his idiocy is on the same level as Hijikata, and he's supposed to be one of the smartest characters, brains of the Shinsengumi and all. But I think everyone on the show is kind of an idiot, except Takasugi, and the dead serious characters.

Sakamoto and Kondo's idiot moments many of you guys know about it... katsura's moments of idiocy...
- Yellow Curry Ninja
- When he and Gin was taking a driving test. (The most idiotic moment I have ever seen.)
- Gin and the baby when Katsura thought that they hid in a can.
- When he suggested the gay opera in episode 50.
- Episode 115.

He's just conserving intellectual energy for when he really needs it, like nabe battles. Plus, if you're an idiot, people don't expect much from you so you avoid the stress of having to meet high expectations AND you avoid disappointing people by not meeting those expectations because they don't exist in the first place. It's a win-win situation.

I don't think he's stupid, it's just apathy, boredom and laziness, he understands a lot more than most of the 'clever' chars and tends to get to the heart of the matter of how people feel or think before anyone else. It's his attitude and choice of words that get him labled stupid, and hit a lot XD